The invention relates to a tuning circuit of the type described in the periodical "Funkschau" 1976, volume 5, pages 166 et seq.
In this known tuning circuit, in addition to the means for digitally controlled turning, a circuit operating in analog manner is provided for automatic fine tuning (AFC circuit).
In circuits for automatic fine tuning of a receiver with residual sideband modulation, e.g. for a television receiver, quite different conditions obtain from those with a radio receiver which is suitable for the reception of transmitted signals in which both sidebands are transmitted. The received carrier does not lie at the middle of the selection curve. From this there results an asymmetrical capturing and holding range of the AFC circuit (See Funkschau 1959, volume 20, page 489).
In the digital storing of encoded values of tuning voltage and the obtaining of the related direct tuning voltage by means of a digital/analog converter (D/A converter), the problem exists that after a certain time the value of direct voltage adjusted with a definite code word may deviate from the original value owing to drift phenomena. This is e.g. the case when the value with the apparatus warmed up is stored and the stored value has to be adjusted again with the apparatus cooled down. Since the range of capture of the AFC circuit in one direction is relatively small there is the danger that upon a drift in this direction the transmitter associated with the stored code word is no longer identified and captured. Upon a drift in the other direction the capturing range of the AFC circuit, in contrast, has a sufficiently large reserve.
This danger is increased further when in the storing of the values of the tuning voltage it is not the value corresponding to exact tuning that is stored but an optimum value in accordance with the subjective impression of the operator, which value is only a little way from the limit of the capturing range of the AFC circuit. Such a desired wrong tuning is possible when the frequency discriminator of the AFC circuit is adversely affected by an additionally stored code word.